Olivia: Lose a bet?
Peter: It was either that or flying lessons.

It’s a testament to how far Peter and Walter’s relationship has come since the pilot that Peter actually lets Walter drive their car at the beginning of this episode. Despite a few occasional disagreements, they’ve become completely comfortable around each other, and Peter now sees Walter’s quirks as endearing rather than frustrating. When Walter suggests that Peter and Olivia should get married, for example, Peter reacts not with frustration, but with a mixture of resignation and amusement.1 But that state of comfort is about to be tested.

Ten years earlier, when Walter was stuck in St. Claire’s and his relationship with his son was at its nadir, Peter decided to sell his grandfather’s old books, which had been left to Walter, out of spite. Little did he know that a formula for a customizable toxin could be scribbled somewhere in the books’ margins. When Walter finds out about this, he is furious. Not only did Peter sell something of great sentimental value to him, he also gave away classified research to people who might exploit it for evil purposes.

Thankfully, the formula wasn’t written in any of the books, absolving Peter of any guilt caused by being a potential accessory to the evil doings of this episode’s villain. However, his relationship with his father has still been put in a precarious position. Peter needs to make it up to Walter somehow. So he recovers the books from the artist who had purchased them and gives them back to Walter. Some of them have been destroyed, but Walter nonetheless appreciates the symbolism of the gesture.

To Walter, there is nothing more important than family. With his wife dead and his academic career ruined, he really has nothing else left to live for. That’s why any perceived betrayal from his son stings so badly. But Walter forgives Peter easily, and that may be because he’s harbouring some secrets of his own. After killing this episode’s villain, he tells Olivia, “There’s nothing I wouldn’t do,” as if he’s on the verge of spilling those secrets, but in the end, he holds back. However, he won’t be able to hold back forever. Eventually, those secrets will be revealed, and when they are, Peter may be the one feeling betrayed.

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1 Of course, that may be because Peter actually does want to marry Olivia, but let’s just leave any discussions of their relationship to later commentaries. ^